NH 3 on anatase TiO 2 ( 101 ) : Diffusion mechanisms and the effect of intermolecular repulsion

We utilized scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations to study the diffusion of ammonia $({\mathrm{NH}}_{3})$ on anatase ${\mathrm{TiO}}_{2}(101)$. From time-lapsed STM imaging, we observed monomeric and dimeric diffusion channels, and a general tendency to higher diffusion rates with increasing ${\mathrm{NH}}_{3}$ coverage. In surface regions where several ${\mathrm{NH}}_{3}$ molecules are adsorbed within a few sites, we further observed the diffusion of ${\mathrm{NH}}_{3}$ molecules occurring in cascades, where the diffusion of one adsorbate triggers that of others. This eventually leads to apparent diffusion barriers that are lower than expected within a single-jump model. From the DFT calculations, we obtained mechanistic insights into the two observed ${\mathrm{NH}}_{3}$ diffusion channels. Within the dimeric ${\mathrm{NH}}_{3}$ diffusion channel, one ${\mathrm{NH}}_{3}$ swings around another adsorbed ${\mathrm{NH}}_{3}$ and experiences a reduced diffusion barrier, owing to the intermolecular bonding during the event.